A secondary battery which has a high applicability to a product group and an electrical property such as a high energy density is widely applied not only to portable equipment, but also an electric vehicle (EV) or a hybrid vehicle (HV) which is driven by an electrical driving source or an energy storage system (ESS) or an uninterruptible power supply (UPS) system which uses a medium or large size battery used for domestic or industrial purpose.
Such a secondary battery has a primary advantage which drastically reduces the usage of a fossil fuel but does not create by-products in accordance with the usage of the energy, so that the secondary battery has drawn attention as a new energy source which is environmentally friendly and improves energy efficiency.
When the secondary battery is embodied as a battery for a portable terminal, suitability of the secondary battery for a high capacity environment is not necessarily increased. However, the battery which is applied to the electric vehicle or an energy storage source is generally used such that a plurality of unit secondary battery cells are grouped together to increase suitability for a high capacity environment.
When such a battery, specifically, a plurality of secondary batteries is alternately charged and discharged, it is required to manage the battery such that the charging and the discharging of the batteries are efficiently controlled to maintain appropriate operation status and performance.
To this end, a battery management system (BMS) which manages the status and the performance of the battery is provided. The BMS measures a current, a voltage, or a temperature of the battery to estimate a state of charge (SOC) of the battery based on the measured current, voltage, and temperature and controls the SOC so that the fuel consumption efficiency is the best. In order to correctly control the SOC, it is required to correctly measure the SOC of the battery which is being charged and discharged.
A method which measures an SOC of the battery in the BMS of the related art includes a method which accumulates a charged/discharged current which flows in the battery to estimate an SOC of the battery. However, according to this method, an error which is generated when a current is measured by a current sensor is continuously accumulated so that accuracy of the SOC is lowered as time goes by.
Alternatively, there is a method which measures a voltage of a battery while charging/discharging the battery, estimates an open circuit voltage (OCV) of the battery in a non-loaded status from the measured voltage and maps the SOC corresponding to the estimated open circuit voltage referring to an SOC table for every open circuit voltage. However, the voltage which is measured while charging or discharging the battery is largely different from an actual voltage. For example, when the battery is connected to a load to start discharging the battery, the voltage of the battery is suddenly lowered and when the charging of the battery is started from an external power source, the voltage of the battery is suddenly increased. Accordingly, accuracy of the SOC may be lowered due to an error between the voltage measured at the time of charging/discharging the battery and the actual voltage.
Alternatively, a method which considers the battery as an electrical model and compares a theoretical output value of the battery model with an actual output value to estimate the SOC through active correction includes an SOC estimating method which is called an extended Kalman filter (EKF) SOC estimation algorithm. According to the EKF SOC estimation algorithm, a maximum error of the SOC is low, for example, 3% at a room temperature and the SOC estimation may be stably performed significantly regardless of the power pattern so that the EKF SOC estimation algorithm is widely used to estimate the SOC of the battery.
The EKF SOC estimation algorithm extracts a parameter of the battery model through an experiment and makes the extracted parameter as a table. However, the parameter which has been extracted and calculated is not changed so that the accuracy of the SOC is high at an initial stage but the accuracy of the SOC may be lowered in accordance with actual storage status and the degradation of the battery.